Practice question

DNA containing BrdU substituted in place of thymidine becomes photosensitive because carbon-bromine bond cleaves homolytically upon absorption of near-UV light around 313 nm or visible light when sensitized by intercalating dyes Hoechst 33258 or acridine orange. Resulting uracilyl radical abstracts hydrogen from deoxyribose causing strand break at apurinic site, creating alkali-labile lesions and single- and double-strand breaks that overwhelm base excision repair leading to cell death if incorporation extensive. This property underlies selective elimination technique: pulse culture with BrdU labels rapidly dividing cells heavily, while quiescent cells incorporate minimally. Subsequent exposure to bright fluorescent light or UV delivers lethal damage specifically to BrdU-rich DNA, sparing non-dividing population. Heat, acid washing, or cold shock do not exploit bromine photochemistry, thus ineffective for selective killing. Method known as light-induced suicide historically enabled enrichment of stem cells, isolation of cell cycle mutants, and purification of G0 populations for transplantation studies where removal of proliferative fraction critical.